The atomic mass of carbon is about The different isotopes of carbon do not differ appreciably in their chemical properties.
The activity of the modern radiocarbon standardis about 14 disintegrations per minute dpm per gram carbon.
Once an organism dies the carbon is no longer replaced.
Because the radiocarbon is radioactive, it will slowly decay away.
The technique was developed by Willard Libby and his colleagues in during his tenure as a professor at the University of Chicago.
Libby estimated that the steady state radioactivity concentration of exchangeable carbon would be about 14 disintegrations per minute dpm per gram.